The 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as “LTE”), and LTE-Advanced, which is an evolved version of LTE (hereinafter, referred to as “LTE-A”), employ orthogonal frequency division multiplexing (OFDM) scheme for the downlink communication scheme.
In the OFDM scheme, studies are being carried out on employing of adaptive modulation, frequency scheduling or the like for each resource block (RB), which bundles a plurality of subcarriers, in order to improve the frequency utilization efficiency. Adaptive modulation is a technique of determining a coding rate and a modulation scheme that satisfy a predetermined packet error rate in accordance with a channel state observed on the receiving side. Frequency scheduling is a technique whereby a plurality of terminals (which may also be called UE (User Equipment)) report channel states to a base station (which may also be called cell, eNB or nodeB) for each RB using reference signals from the base station, and the base station gathers the channel states and allocates appropriate RBs to each terminal according to a predetermined scheduling algorithm (see FIG. 1).
The values used to report the channel states used in adaptive modulation, frequency scheduling or the like are called channel information (CSI). CSI includes CQI (Channel Quality Indicator). As an example of operation, a terminal calculates an index corresponding to the channel state (receiving quality in particular) as an SINR (Signal to Interference plus Noise ratio:power ratio of interference and noise to a desired signal) and reports a set of a coding rate and modulation scheme that satisfy a predetermined packet error rate as an index (e.g., CQI) on a predetermined table.
Introduction of coordinated multiple transmission point (CoMP) is under study. CoMP is a technique whereby a plurality of base stations (transmission points (TP)) cooperate with each other to transmit signals to a terminal (UE), and several schemes are under study. For example, two main CoMP schemes are under study in 3GPP: (1) CB (coordinated beamforming) scheme and (2) JT (Joint Transmission) scheme.
The CB scheme is a scheme in which only a specific TP stores data intended for a certain terminal. That is, a signal from a TP that stores no data intended for the terminal (e.g., TP adjacent to a TP to which the terminal is connected) is regarded as interference to the terminal. The CB scheme adopts a method of reducing inter-TP interference through control of transmission parameters. More specifically, examples of transmission parameters include precoding, transmission power, modulation scheme and coding rate. Appropriately controlling these transmission parameters makes it possible to weaken signals from an interference TP (TP that possesses no data intended for the terminal) for the terminal while strengthening signals from a desired TP (TP that possesses data intended for the terminal). Strengthening signals from a desired TP and weakening signals from an interference TP may contradict each other depending on the circumstances, but various proposals are being made taking into account the trade-off between the two.
On the other hand, the JT scheme is a scheme in which data to a certain terminal is shared by a plurality of TPs. Thus, a plurality of TPs can simultaneously transmit signals intended for the terminal. For this reason, since the terminal can handle signals from other TPs not as interference signals but as desired signals, an SINR observed at the terminal can be expected to improve. Furthermore, improving a method of generating precoding weights at a plurality of TPs as an operation within a network allows for an even greater performance improvement.
For such CoMP control, there is a method of observing channel information between CoMP control target TPs and a terminal and reporting the channel information to the network as channel information in units of TPs.
As reference signals used to measure or report CSI during CoMP control, there is a CSI-RS (Channel State Information Reference Signal: reference signal for measurement of channel information). FIGS. 2A to 2C illustrate configuration examples of CSI-RS with respective numbers of transmitting antenna ports. As shown in FIGS. 2A to 2C, CSI-RSs are defined by a configuration corresponding to the number of transmitting antenna ports (8 ports, 4 ports or 2 ports) of the base station. In FIGS. 2A to 2C, one RB consists of 12 subcarriers and each block shown in FIGS. 2A to 2C represents resources of two OFDM symbols in each subcarrier that are continuous in the time domain (2 REs (Resource Elements)). In each block (2 REs) shown in FIGS. 2A to 2C, CSI-RSs corresponding to two ports are code-multiplexed.
Each terminal acquires information relating to CSI-RS from the base station beforehand. More specifically, the information relating to CSI-RS includes, for example, the number of antenna ports (antennaPortsCount), subcarriers within a subframe, and CSI-RS configuration number that identifies an OFDM symbol position (resourceConfig, hereinafter may be represented by “CSI-RS config(i)” or “#i,” CSI-RS configs (0) to (19) in FIGS. 2A to 2C), transmission subframe configured of a transmission period and an offset (subframeConfig), and a power ratio (p-C) between reference signals and data signals (see NPLs 1 and 2).
In FIGS. 2A to 2C, CSI-RS configuration numbers are assigned in order in the time direction and in order in the frequency direction at the same point of time. Moreover, as shown in FIGS. 2A to 2C, the same number is assigned to resource starting positions of the respective CSI-RS configuration numbers (starting positions in order of number assignment) between CSI-RS configurations corresponding to the respective numbers of antenna ports. As shown in FIGS. 2A to 2C, a CSI-RS configuration used when the number of antenna ports is small constitutes a subset of a CSI-RS configuration used when the number of antenna ports is large (may also be called “(nested structure”). It is thereby made possible to identify the resources for each of the number of antenna ports with minimum numbers while using an overlapping number in the CSI-RS configuration corresponding to each of the numbers of antenna ports. For example, CSI-RS config(0) with two ports shown in FIG. 2C can be identified as only resources corresponding to two ports (2 REs) from the starting position of CSI-RS config(0) with eight ports (8 REs) shown in FIG. 2A.
Note that a procedure for a base station to indicate, to a terminal, information relating to CSI-RS of each TP beforehand is adopted to observe channel information between CoMP control target TPs (hereinafter represented by “coordinating TP” or may also be called “CoMP measurement set”) and a terminal.
There is also a muting technique that makes data of the TP to which the terminal is connected a non-transmission signal in order for the terminal to observe reference signals (CSI-RSs) transmitted from peripheral TPs among coordinating TPs (CoMP measurement set). More specifically, each of CSI-RS configs (0) to (9) (see FIG. 2B) which are 4-port CSI-RS configuration numbers of the aforementioned CSI-RS configuration numbers is expressed in a bitmap, and the base station indicates, to the terminal, which resource is designated as a non-transmission signal resource. The information on the bitmap type indicating which resource is designated as a non-transmission signal resource is called “non-transmission CSI-RS configuration number list” (zeroTxPowerResourceConfigList) (see NPL 2).
For example, when resources of CSI-RS configs (1) and (2) of CSI-RS configs (0) to (9) are designated as non-transmission signal resources, the non-transmission CSI-RS configuration number list becomes {0, 1, 1, 0, 0, 0, 0, 0, 0, 0}. In the list, “1” represents a non-transmission signal resource and “0” represents a resource other than a non-transmission signal resource in correspondence with CSI-RS configs (0) to (9) respectively in order from the leading bit of the non-transmission CSI-RS configuration number list.
The base station indicates, to the terminal, a transmission subframe as well (zeroTxPowerSubframeConfig) configured of a transmission period and an offset like the aforementioned CSI-RS as a subframe in which a non-transmission signal resource is configured. This allows the terminal to identify which resource in which subframe becomes a non-transmission signal resource.
FIG. 3 illustrates positions of non-transmission signal resources (CSI-RS configs (1) and (2)) within a subframe corresponding to zeroTxPowerSubframeConfig configured in a TP to which a certain terminal is connected. In this case, by causing the CSI-RS configuration of a TP positioned in the periphery of the TP to associate with any one of non-transmission signal resources (CSI-RS config(1) or (2) in FIG. 3), the terminal no longer receives interference from data from the TP to which the terminal is connected and can secure CSI measuring accuracy when observing CSI-RSs of the peripheral TP as desired signals.